1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and a method for producing the same.
2. Description of the Related Art
The liquid crystal display apparatus has been put into practical use in a wide range of applications from hand-held calculators to portable television. In Particularly, an active matrix type display apparatus in which each pixel of a display section has a switching element is often used when a clear image display is required.
In the active matrix type display apparatus, a thin film transistor (hereinafter referred to as "TFT") using a thin film of amorphous silicon, polysilicon or the like is used as the switching element. The characteristics of the TFT depend on the materials of a thin film constituting the TFT. The materials of the thin film of the TFT can be broadly classified into three types as follows:
(1) amorphous silicon PA1 (2) low-temperature polysilicon PA1 (3) high-temperature polysilicon PA1 Mobility: 1500 cm.sup.2 V.sup.-1 S.sup.-1 PA1 ON/OFF current ratio of the transistor: 10.sup.9 or more PA1 Maximum operating frequency of the transistor: several GHz PA1 forming a conductive film on the protection film; polishing the conductive film so as to mirror finish the surface of the conductive film, forming the gaps on the conductive film so as to form the plurality of pixel electrodes in a matrix; coating the entire surface of the first substrate with the filler so as to fill the gaps and cover the pixel electrodes; and performing a planarization process and an alignment process on the filler at the same time.
Hereinafter, thin films using the above silicons are described, respectively.
(1) Since an amorphous silicon thin film can be formed at a low temperature of around 350.degree. C., common inexpensive glass such as Corning 7059 (manufactured by Corning Co., Ltd.) can be used as a substrate on which the thin film is formed. The use of the amorphous silicon thin film makes it possible to realize a relatively large liquid crystal display apparatus having a diagonal length of 15 inches. Such a liquid crystal display apparatus is now the main type used in active matrix type liquid crystal display apparatus. However, common inexpensive glass cannot be processed at a temperature equal to or higher than 600.degree. C. Therefore, a thermal oxide film that is excellent in electrical insulation where pin holes are not likely to occur cannot be obtained on the substrate formed of the common inexpensive glass. Moreover, there exists a number of capture levels in the thin film made of the amorphous silicon, and the field effect mobility .mu.e (electron mobility) is approximately 0.1 to 0.5 cm.sup.2 V.sup.-1 S.sup.-1. Therefore, since the TFT using the amorphous silicon thin film has a relatively high ON-resistance, a circuit, such as a driver circuit, including fine transistors having an excellent performance cannot be fabricated on the same substrate on which the display section is formed. Thus, a driver IC should be mounted on the substrate using a film carrier or the like.
However, this mounting of the driver IC raises a new problem. In the case of a projection-type LCD having a size of approximately 2 inches, the pitch of pixels of the display section is 21 .mu.m vertically and 23 .mu.m horizontally. In the case of a color display by a color filter, the pitch is reduced to 21 .mu.m vertically and 8 .mu.m horizontally. At present, it is impossible to connect the driver IC to each line for the pixels with the fine pitch, and as a result, the driver IC cannot be applicable to a projection-type LCD.
(2) A low-temperature polysilicon is formed by the crystallization of silicon by means of an annealing process or a laser annealing process conducted over a long period of time. Heat resisting glass is used for the substrate on which a thin film of the low-temperature polysilicon is formed. In this case, the maximum processing temperature is in the range of 550.degree. to 600.degree. C. In the case of the low-temperature polysilicon, a field effect mobility .mu.e is approximately 50 cm.sup.2 V.sup.-1 S.sup.-1 and .mu.h (hole mobility) is approximately 15 cm.sup.2 V.sup.-1 S.sup.-1. Thus, in general, the low-temperature polysilicon TFT has better transistor characteristics than the amorphous silicon TFT.
(3) A high-temperature polysilicon is formed utilizing a quartz substrate excellent in heat resistance. Since the quartz substrate can be processed at a high temperature of approximately 1000.degree. C., the fabricating process of the IC can be adopted to the fabrication of the high-temperature polysilicon TFT. Among the three methods mentioned above, this method makes it possible to form the TFT having the best characteristics. A field effect mobility .mu.e of the high-temperature polysilicon is approximately 100 cm.sup.2 V.sup.-1 S.sup.-1.
As described above, since a better transistor can be obtained with the polysilicon than with the amorphous silicon the polysilicon is advantageous in that the TFT and part of the driving circuit can be integrally formed on the glass substrate.
However, the TFT formed of the low-temperature polysilicon has a problem of a low operating speed. For example, in the case where a CMOS shift resister is formed using the low-temperature polysilicon TFT, the maximum operating frequency is measured to be no more than approximately 5 MHz.
The field effect mobility e of the high-temperature polysilicon is relatively high as compared with those of the amorphous silicon and the low-temperature polysilicon. Therefore, the TFT formed of the high-temperature polysilicon is excellent in characteristics with the maximum operating frequency of approximately 15 MHz.
The development of an HDTV (High Definition TV) with high precision is urgently required as the next generation visual medium.
A display for an HDTV has 1125 scanning lines, 1875 data lines, and about 2,100,000 pixels corresponding to these lines. In the case of a color display, the number of the pixels is tripled, i.e, about 6,300,000 pixels.
In the case of a non-interlace driving, the field frequency is 60 Hz, the driving time for each scanning line is about 15 microseconds, and the frequency of the each scanning line is 67.5 KHz. The driving time for each data line is about 0.008 microseconds without a color filter, and the frequency of each data line is about 130 MHz. In the case of a color display, the frequency is 390 MHz.
In the case of an interlace driving, the field frequency is 60 Hz, the number of scanning lines is 560, and the driving frequency of each scanning line is 33.75 KHz. The driving frequency of each data line is about 63 MHz without a color filer. In the case of a color display, the frequency is 190 MHz.
The above driving frequency cannot be obtained with the operating speed of the polysilicon TFT described above. In order to obtain a driving frequency necessary for the HDTV, data processing such as a time-scale expansion is required, resulting in a complicated peripheral circuit system.
Moreover, since the leak current is large in the polysilicon TFT, it is impossible to make the ratio of an ON current to an OFF current(ON/OFF current ratio) large. The ON/OFF current ratio of the polysilicon TFT is approximately 10.sup.7. In order to be applicable to the HDTV, the ON/OFF current ratio should be about 10.sup.8. For improving the ON/OFF current ratio, it is necessary to make the TFT larger in size or to connect the TFTs in series. However, with such a modification, there arises a new problem in that an LCD cannot be miniaturized.
As a method for solving such a problem, the method of mounting the TFTs on a single-crystalline silicon substrate has been disclosed. The transistor characteristics in the case where the TFTs are formed on the single-crystalline silicon are as follows:
As seen from the above description, if a single-crystalline silicon is used as the substrate of the display apparatus and the method of forming the transistors on the single-crystalline silicon is employed, the operating speed of the transistors required for the display apparatus for the HDTV can be realized.
FIG. 5 shows an example of a liquid crystal display apparatus in which switching transistors are formed on the substrate made of single-crystalline silicon. As shown in FIG. 5, a field silicon oxide film 56 is formed on an entire surface of a base substrate 57 made of single-crystalline silicon. The field silicon oxide film 56 has opening areas for defining active regions of the transistors. A source region 58 and a drain region 59 of each transistors are formed in the corresponding active region of the base substrate 57. Each MOS transistor has a gate insulating film 61 and a gate electrode 60. The source region 58 and the drain region 59 are in contact with aluminum electrodes 54a and 54b formed on the field silicon oxide film 56, respectively.
The switching transistors are covered with a protective film 55 formed on an entire surface of the base substrate 57. The protective film 55 has a through hole 55b in each unit pixel region, reaching the aluminum electrode 54b. On the surface of the protection film 55, a plurality of pixel electrodes (also used as reflectors) 54 divided by a gap 55a are formed in a matrix so as to cover substantially the entire surface of the base substrate 57. The pixel electrode 54 is connected with the aluminum electrode 54b via the through hole 55b. The pixel electrode 54 is made of aluminum.
A transparent counter electrode 52 is formed on the entire counter surface of the glass substrate 51 disposed so as to face the base substrate 57. An alignment film (not shown in FIG. 5) is formed so as to cover the counter electrode 52.
Since the single-crystalline silicon is opaque, it is used as a substrate for a reflection-type display apparatus. Therefore, a problem which does not occur in a transmission-type display apparatus arises.
The reflection-type liquid crystal display apparatus used for a projection-type display system requires the following conditions:
(1) In the reflection-type liquid crystal display apparatus, reflectors (also used as pixel electrodes) are formed on one of the substrates. Surfaces of the reflectors should be mirror finished.
(2) The reflectors should be separately provided for each pixel. In order to make the alignment of the liquid crystal molecular uniform, the surface of a through hole provided between the reflectors should be flush with the surfaces of the reflectors.
In order to meet the above requirements, Japanese Laid-Open Patent Publication No. 56-83781 discloses an invention as described below. FIG. 6 shows a sectional view of a liquid crystal display apparatus according to the disclosed invention.
In the disclosed display, the surface of a substrate is coated with polyimide 18 so as to be planarized. After a through hole 19 is made at a required portion, it is filled with metal 20, so that surfaces of the pixel electrodes 21 can be planarized. However, the surfaces of the pixel electrodes 21 are not mirror finished in this method, therefore the disclosed display apparatus is insufficient as a projection-type liquid crystal display device.
Japanese Laid-Open Patent Publication No. 53-72647 discloses three methods as described below. FIG. 7 shows a sectional view of a substrate of a liquid crystal display apparatus according to the disclosed invention.
(1) A method including the steps of: forming a planarization film 71 on an entire surface of the substrate 70; forming a contact hole to one of the electrodes of a transistor previously formed on the substrate 70; and forming a reflector 74 on the entire surface of the substrate 70 by vapor-deposition.
(2) A method including the steps of: forming the planarization film 71 on an entire surface of the substrate 70; forming a contact hole to one of the electrodes of a transistor previously formed on the substrate 70; filling a through hole 72 with metal 73; and forming the pixel electrode 74 by vapor-deposition.
(3) A method including the steps of: coating the surface of the substrate 70 with a photoresist; forming a contact hole to one of the electrodes of a transistor previously formed on the substrate 70; filling a through hole 72 with metal 73; removing the photoresist; coating the substrate 70 with the planarization film 71; polishing the entire surface of the planarization film 71; and forming the reflector 74.
In any of the methods described above, after the formation of the metal layer as a pixel electrode, mirror finishing is not performed on the surface of the metal layer. Therefore, the minute unevenness (hillocks) on the metal layer produces scattering of light. Thus, the resultant display apparatus is insufficient as a projection-type liquid crystal display apparatus.
Moreover, since the smoothing process of the surfaces of the fillers for filling gaps separating the pixels and the surfaces of the pixel electrodes is not performed, incomplete alignment of the liquid crystal molecular occurs.
The methods described above were originally directed to the application of a direct-view type display in which the requirements of smoothing the surface of the substrate and mirror finishing the pixel electrodes were not as strict as those of the projection type display.
FIG. 8 shows a sectional view of a substrate of a liquid crystal display apparatus disclosed in Japanese Laid-Open Patent Publication No. 57-122479. In this display apparatus, polycrystalline silicon 22 is formed on a driving transistor section formed on the substrate. Then, after planarizing an entire surface of the polycrystalline silicon 22, pixel electrodes 30 are formed thereon. Therefore, the surface of the polycrystalline silicon 22 is planarized. However, since mirror finishing is not performed on the surface of the pixel electrode 30 made of metal formed on the polycrystalline silicon 22, scattering of light due to the hillocks of metal occurs. Thus, the resultant display apparatus is insufficient as a projection-type liquid crystal display apparatus. Since the smoothing process of the surfaces of the fillers 29 for filling gaps 27 separating the pixel electrodes 30 and the surfaces of the pixel electrodes 30 is not performed, incomplete alignment of the liquid crystal molecular occurs.
The objectives of the present invention are to provide a projection-type liquid crystal display apparatus having a substrate of which the reflecting face is smoother than a conventional one and a method for producing the same, thereby contributing to the realization of a projection-type liquid crystal display apparatus applicable for the HDTV.